Patch up docs reflect 9.1 is not supported in 2.3

[robe]

Since PostgreSQL 9.1, is going to reach EOL probably before 2.3 release, it's time to kill it as noted in

​https://lists.osgeo.org/pipermail/postgis-devel/2015-November/025480.html

Couple of things need changing, some for 2.2 as well

1) our header in docs says if compiling with less than 9.1 (that header should be changed to compiling without raster support) since now it's only relevant if you are compiling without raster support or you for whatever reason want to use scripts.

​http://postgis.net/docs/manual-2.2/postgis_installation.html#create_new_db

This should really read without extension support (no raster)

So it's clearer to folks installing wo raster that they need to use these instructions.

2) Need to update minimum requirements in docs

We should start looking thru our code, to see what stuff we can leverage that is only supported in 9.2+ and take out any ifDEF stuff targetting < 9.2. That can be saved for later as a separate ticket.

3) Also change our configure to not allow compiling against lower than PostgreSQL 9.2, because backend guc stuff and raster gets pretty nasty on 9.1

[robe]

In 15030:

Mark PostgreSQL 9.2 as minimum supported
closes #3384